1. Field of the Invention
The present invention relates to a secondary battery in which a plurality of electrode terminals extending from an end of an electrode sheet are connected to the back of a lid element through one collecting terminal, and a manufacturing method thereof.
2. Description of the Related Art
Electric cars or hybrid cars have been developed in consideration of environmental issues, and compact and lightweight secondary batteries with high performance are needed. Such secondary batteries include a lithium-ion battery, for example. The lithium-ion battery has a small size and a light weight, a large capacity, and good charging characteristics and good cycle characteristics.
A prior art of such a secondary battery is hereinafter described with reference to FIG. 1 and FIG. 2a to FIG. 2f. FIG. 1 is a vertical sectional view showing the inner structure of a conventional secondary battery, and FIG. 2a to FIG. 2f are diagrams for explaining a method of manufacturing the conventional secondary battery.
As shown in FIG. 1, secondary battery 100 herein illustrated comprises case 101 in which electrode element 102 is accommodated. Case 101 is shaped into a hollow cylinder with its bottom closed and its top opened, and is made of iron which is a conductive material.
Electronic element 102 comprises a positive pole sheet, a separator sheet, a negative pole sheet and hollow core 103, and is configured such that the positive pole sheet, the separator sheet and the negative pole sheet are laminated and wound around core 103 in cylindrical shape. A plurality of electrode terminals 104 and 105 are provided to protrude at a plurality of points on the upper edge of the positive pole sheet and on the lower edge of the negative pole sheet, respectively. Thus, these positive/negative electrode terminals 104 and 105 extend from a plurality of points on the top and the bottom of electrode element 102, respectively.
Positive pole electrode terminals 104 extending upward from the positive pole sheet are bound into one, and one collecting terminal 106 is welded to the upper end thereof. Insulating tape 107 is wound around the upper portion of electrode terminals 104 thus bound into one and the lower portion of collecting terminal 106. The upper end of collecting terminal 106 which is not wound in insulating tape 107 is welded to the back of lid element 108.
Lid element 108 is formed in disk shape with aluminium which is a conductive material, and is integrally provided at its outer edge with gasket 109 made of resin which is an insulating material. Since gasket 109 is fixed to the top end of case 101, the opening on the top of case 101 is closed by lid element 108 in an insulating state.
Negative pole electrode terminals 105 extending downward from the negative pole sheet are bent toward the center of electrode element 102. At the center, all electrode terminals 105 are disposed one on another and directly welded to the inner surface of case 101 on the bottom. In electrode element 102, the gaps between every sheet are impregnated with a nonaqueous electrolytic solution (not shown).
In secondary battery 100 as configured above, the positive pole sheet in electrode element 102 impregnated with the nonaqueous electrolytic solution produces a positive electric potential and the negative pole sheet produces a negative electric potential. Thus, lid element 108 on the top serves as a positive pole and the bottom of case 101 serves as a negative pole.
Description is now made in brief for a method of manufacturing secondary battery 100 as configured above. The positive pole sheet connected with electrode terminals 104 at the plurality of points on the upper edge, the separator sheet, and the negative pole sheet connected with electrode terminals 105 at the plurality of points on the lower edge are laminated and wound around core 103 to form circular cylindrical electrode element 102 as shown in FIG. 2a.
Next, as shown in FIG. 2b, positive pole electrode terminals 104 extending from the plural points on the upper edge of electrode element 102 are bound into one to which the lower end of one collecting terminal 106 is resistance welded as shown in FIG. 2c. It should be noted that while FIG. 1 shows two electrode terminals 104 and 105 and FIG. 2a to FIG. 2f show three for simplifying illustration, a number of electrode terminals 104 and 105 are actually used.
Next, as shown in FIG. 2d, insulating tape 107 is wound around the upper portion of electrode terminals 104 bound into one and the lower portion of collecting terminal 106. As shown in FIG. 2e, electrode element 102 for which the winding is completed is accommodated into case 101 from the opening on the top.
At this time, negative pole electrode terminals 105 extending from the bottom of electrode element 102 are bent toward the center and disposed one on another, and in this state, abutted against the bottom of case 101. A welding electrode (not shown) is inserted into a through hole in the center of core 103 to perform resistance welding of negative pole electrode terminals 105 disposed one on another on the bottom of case 101.
Next, the upper end of collecting terminal 106 which is not wound in insulating tape 107 is resistance welded to the back of lid element 108, and as shown in FIG. 2f, lid element 108 is fit and fixed to the opening on the top of case 101. The aforementioned method of manufacturing secondary battery 100 can be changed in the order of the steps as long as the contents cause no contradiction.
For example, it is possible that electrode element 102 is accommodated into case 101, and then, electrode terminals 104 are bound into one, collecting terminal 106 is welded thereto, and insulating tape 107 is wound around them. In addition, it is not impossible that the upper end of collecting terminal 106 is connected to the back of lid element 108 in advance and the lower end of collecting terminal 106 is welded to electrode terminal 104.
In secondary battery 100 as configured above, since positive pole electrode terminals 104 extending from the plurality of points on the top of electrode element 102 are connected to lid element 108 through one collecting terminal 106, lid element 108 can serve as an electrode. In particular, since lid element 108 is connected to collecting terminal 106 with low resistance which in turn is connected to positive electrode terminals 104, a large current can flow from electrode element 102 to lid element 108.
As described above, positive electrode terminals 104 extending from the plural points on the top of electrode element 102 wound in cylindrical shape are bound into one as shown in FIG. 2a. As a result, some of electrode terminals 104 are twisted at angles at which they are strained.
Thus, electrode terminals 104 may be broken. To avoid this, long electrode terminals 104 must be formed, which causes the difficulty of automatically binding electrode terminals 104 into one. In addition, since electrode terminals 104 bound into one is welded to one collecting terminal 106, the welding is complicated and thus a welder (not shown) therefor is required.
The formation of long electrode terminals 104 as mentioned above requires the winding of insulating tape 107 such that they cause no short circuit to the inner surface of case 101, but this task is complicated and automation is difficult. Furthermore, long electrode terminals 104 as mentioned above make it difficult to reduce the gap between the top of electrode element 102 and the bottom of lid element 108, thereby obstructing a smaller size and lighter weight of secondary battery 100.
Additionally, in secondary battery 100 as configured above, since electrode element 102 accommodated in case 101 cannot be fixed as it is, a dedicated spacer (not shown) is required, for example. This further hinders a smaller size and lighter weight and productivity.